How long is the clay good for? What do I do if I open the package and it is dried out?

The clay is good indefinitely. Until it is fired, it can be reconstituted and reworked into something else.

I had a 20-gram piece that I wasn't happy with. I kept thinking that if I just added one more thing here and there, it would be great. Eighty grams and one year later, it was time to give it up. Because it sat on a bureau in a hot dry room for a full year and because it was so big, it took a week to reconstitute. The piece I made with that reconstituted 80 grams is one of my favorites.